As described herein, "acoustic ink printing" is a direct marking process that is carried out by modulating the radiation pressure that one or more focused acoustic beams exert against a free surface of a pool of liquid ink, whereby individual droplets of ink are ejected from the free ink surface on demand at a sufficient velocity to cause the droplets to deposit in an image configuration on a nearby recording medium. This process does not depend on the use of nozzles or small ejection orifices for controlling the formation or ejection of the individual droplets of ink, so it avoids the troublesome mechanical constraints that have caused many of the reliability and picture element ("pixel") placement accuracy problems that conventional drop-on-demand and continuous-stream ink jet printers have experienced.
Several different droplet ejector mechanisms have been proposed for acoustic ink printing. For example, (1) Lovelady et al. U.S. Pat. No. 4,308,547, which issued Dec. 29, 1981 on "Liquid Drop Emitter," provides piezoelectric shell-shaped transducers; (2) a commonly assigned U.S. Pat. No. 4,697,195, which issued Sep. 29, 1987 on "Nozzleless Liquid Drop Emitters," provides planar piezoelectric transducers with interdigitated electrodes (referred to as "IDTs"); (3) a commonly assigned Elrod et al. U.S., Pat. No. 4,751,530, which issued Jun. 14, 1988 on "Acoustic Lens Arrays for Ink Printing," provides droplet ejectors that utilize acoustically illuminated spherical focusing lens; and (4) a commonly assigned Quate et al. U.S. Pat. No. 5,041,845, which issued Aug. 20, 1991 on "Multi-Discrete-Phase Fresnel Acoustic Lenses and Their Application to Acoustic Ink Printing," provides droplet ejectors that utilizes acoustically illuminated multi-discrete-phase Fresnel focusing lenses.
Droplet ejectors having essentially diffraction-limited, f/1 lenses (either spherical lenses or multi-discrete-phase Fresnel lenses) for bringing the acoustic beam or beams to focus essentially on the free ink surface have shown substantial promise for high quality acoustic ink printing. Fresnel lenses have the practical advantage of being relatively easy and inexpensive to fabricate, but that distinction is not material to this invention. Instead, the feature of these lenses that most directly relates to this invention is that they are designed to be more or less diffraction-limited f/1 lenses, which means that their depth of the focus is only a few wavelengths .lambda.; where .lambda. is the wavelength in the ink of the acoustic radiation that is focused by them. In practice, .lambda. typically is on the order of only 10 .mu.m or so, which means that the free ink surface levels of these high quality acoustic ink printers usually have to be controlled with substantial precision.
Apertured cap structures are economically attractive free ink surface level controllers for acoustic ink printing. As pointed out in the above-referenced Khuri-Yakub et al. '937 patent, an apertured cap structure utilizes the inherent surface tension of the ink to counteract the tendency of the free ink surface level to change as a function of small changes in the pressure of the ink. Thus, for example, an apertured cap structure is useful for increasing the tolerance of an acoustic ink printer to the ink pressure variations that can be caused by slight mismatches between the rates at which its ink supply is depleted and replenished. Furthermore, as taught by the '937 patent, a pressure regulator or the like can be employed for maintaining a substantially constant bias pressure on the ink whenever it is necessary or desirable to increase the precision of the surface level control that is provided by such a cap structure.
The fluid dynamics of the acoustic ink printing process generate a generally circular wavefront ripple wave on the free ink surface whenever a droplet of ink is ejected. The viscosity of the ink hydrodynamically dampens this surface ripple wave as it propagates away from the ejection site. However, in printers that have multiple droplet ejectors, such as those that comprise one or more linear arrays of droplet ejectors for line printing, this hydrodynamic damping generally is insufficient to prevent the ripple waves produced by any given one of the droplet ejectors from interfering with the operation of its near neighboring droplet ejectors.
Accordingly, to avoid this unwanted "crosstalk," a multi-ejector printer advantageously includes a cap structure that has a plurality of spatially distributed apertures that surround the ejection sites of respective ones of the droplet ejectors. A cap structure of this type effectively subdivides the free ink surface of the printer into a plurality of individual ponds of ink, each of which is dedicated to a different one of the droplet ejectors. Ink may flow from pond-to-pond between the ejectors and such a cap structure, but the cap structure acts as a physical barrier for inhibiting surface ripple waves from propagating from one pond to another. In operation, the acoustic beams that are emitted by the droplet ejectors of such a multi-ejector printer come to focus more or less centrally of respective ones of the apertures in the cap structure, so the aperture diameters preferably are at least approximately five times greater than (and, indeed, may be twenty or more times greater than the waist diameters of the focused acoustic beams, thereby preventing the apertures from materially influencing the hydrodynamics of the droplet ejection process or the size of the droplets of ink that are ejected. For example, if the acoustic beams have nominal waist diameters at focus of about 10 .mu.m, the apertures suitably have diameters of approximately 250 .mu.m. These relatively large apertures are practical, even for printers that print pixels on centers that are spatially offset by only a small fraction of the aperture diameter, because the droplet ejectors of these higher resolution printers can be, for example, spatially distributed among multiple rows on staggered centers.
As previously pointed out, prior cap structures of the foregoing type have had essentially round apertures. A round aperture configuration suggests itself because of its circular symmetry. However, it now has been found that the retroreflection of the surface ripple waves from the sidewalls of these round apertures is a limiting factor that interferes with operating acoustic ink printers having such cap structures at higher asynchronous droplet ejection rates. Consequently, an aperture configuration that significantly reduces the effect of such surface ripple waves on the acoustic ink printing process is needed to enable such cap structures to be used as free ink surface level controllers for higher speed, asynchronous acoustic ink printers.